1. Field of the Invention
The present invention concerns friction clutches with a low clutch release force, in particular for motor vehicles, and more particularly the clutch mechanism of a clutch of this kind.
2. Description of the Related Art
In a conventional clutch disposed between a driving shaft and a driven shaft a diaphragm bears against the back of a cover fixed to a rotational drive flywheel to move a pressure plate towards said flywheel, forming a reaction plate, to clamp the friction linings of a clutch friction disc between said pressure and reaction plates.
The drive flywheel is constrained to rotate with a first shaft, for example a driving shaft, and the clutch friction disc has at its inside periphery a hub by which it is constrained to rotate with a second shaft, for example a driven shaft.
The clutch is normally engaged to transmit torque between the driving and driven shafts.
To release the clutch in the case of a push type clutch a clutch release thrust bearing presses axially on the inside ends of fingers of the diaphragm to cause said diaphragm to pivot and to eliminate the force that the diaphragm applies to the axially mobile pressure plate in order to release the friction linings. Torque is then no longer transmitted from the driving shaft to the driven shaft because the friction linings are no longer clamped between the pressure and reaction plates constrained to rotate with the hollow cover.
The cover, the pressure plate and the diaphragm usually form a unitary assembly known as the clutch mechanism that is attached by its cover to the flywheel, elastic tongues which allow axial mobility coupling the pressure plate rotationally to the cover. The diaphragm has a hole in the centre and an outside peripheral part in the form of a Belleville washer extended inwardly by a central part divided into radial fingers by slots. The Belleville washer part of the diaphragm bears on the cover and on the pressure plate. Accordingly, the clutch release device of the clutch, which selectively opposes the action of the clutch engaging spring means, consists of the fingers of the diaphragm on whose inside end the clutch release thrust bearing operates, whereas the Belleville washer of the diaphragm constitutes axially acting clutch engaging spring means for clamping the friction linings between the pressure and reaction plates and therefore urging the pressure plate axially away from the back of the cover.
When unstressed, the diaphragm is frustoconical in shape. When fitted into the clutch its Belleville washer is pre-stressed and is flattened to a greater or lesser degree. The cone angle of the Belleville washer is modified on disengaging the clutch.
The characteristic curve of the diaphragm, which represents the force exerted as a function of the clutch release travel, for example at the inside end of the diaphragm fingers, is determined by the dimensions of its Belleville washer, in particular by the ratio of the height of the frustoconical Belleville washer when unstressed to the thickness of the diaphragm. The characteristic curve has a maximum.
Accordingly, the force to be applied to the inside end of the diaphragm fingers during release of the clutch increases to a maximum, decreases gradually to a minimum and then increases again.
There can be a considerable difference between the maximum force and the minimum force. For more information on the characteristic curve reference may be had to document FR-A-1 392 569, for example.
Document FR-A-1 392 569 describes a progressive action spring device external to the clutch friction disc to eliminate the aforementioned maximum during the clutch release travel. The device is mounted in series with the Belleville washer of the diaphragm and produces a spring force significantly less than that of the Belleville washer. The device has a limited travel between a pre-stressed position in which its force is maximum and a stressed position in which its force is minimum.
This achieves an increasing clutch release force at the clutch release thrust bearing.
Assistance of the required magnitude may not be obtained over the clutch release travel, the progressively acting device reducing the force applied by the diaphragm to the pressure plate during clutch release.
The load curves of the progressively acting device, which is usually mounted within the clutch, and the diaphragm have shapes that prevent them being combined to obtain a high level of clutch release assistance, in particular if the clutch release travel and force are to be compatible with existing clutch release devices. What is more, the shape of the curve of the progressive action device changes very quickly through the service life of the clutch, which eliminates clutch release assistance in direct proportion to the required level of assistance. To prevent ageing of the assistance device curve it is preferable to locate the device where its stiffness curve will remain stable in time and in particular at a location outside the progressive action area where lining contact and indentation change and cause the progressive action curve to change.
Consideration could be given to using an assistance spring in parallel with the diaphragm. In this case the assistance spring, for example in the form of a Belleville washer, could bear on the cover and on the inside end of the diaphragm fingers. In the clutch engaged position it could then apply a minimum force and thereafter, its cone angle varying, a force to assist release of the clutch. The assistance force must therefore be minimum in the clutch engaged position throughout the service life of the clutch.
The ideal for this type of implementation is to use an adjuster device, referred to hereinafter as the wear compensator device, which holds the diaphragm in substantially the same position all the time, regardless of wear of the friction linings of the disc and/or of the friction faces of the pressure and reaction plates, so that under all circumstances and throughout the service life of the clutch the assistance spring applies a very low force when the clutch is in the engaged position.